The present invention relates generally to precision trimming of semiconductor devices, and more specifically relates to semiconductor precision trimming methods and apparatus without requiring a laser for the trimming operation.
Apparatus and methods for precision trimming of semiconductor or thin film devices and circuits are well known in the art. Trimming is often necessary since the absolute-value tolerances associated with such semiconductor or thin film devices are not acceptable, after fabrication, to meet a given design tolerance. Trimming techniques that are known by those skilled in the art include, for example, oxidation, where by heating certain resistor films in an oxidizing atmosphere, some of the material on the film surface is converted to a nonconductive oxide layer to increase the total resistance value, and annealing, which causes the grain structure of a device to reorient itself in a more dense fashion thereby reducing the sheet resistance of the device. Another common trimming method employs narrow fusible links between prearranged segments or taps of a component, such as a resistor. These links are typically formed of metal (e.g., aluminum) and initially short-circuit all taps together, but they can be selectively open-circuited by burning them out, either by external means (e.g., laser) or internal means, for example by passing an excessive amount of current through the particular link(s) to be blown.
One application which highlights the importance of precision trimming is the present manufacture of pin electronics used in automated test equipment (ATE) systems and the like, which rely primarily on laser trimming methods and apparatus to meet necessarily precise design performance specifications. Driver circuits utilized in pin electronics, for example, are required to have well-defined output slew rates (i.e., the rate of change of a signal), and because process variations are too great to accurately predict the absolute value of a given reference signal, laser trimming is commonly used. Laser trimming, however, has several inherent disadvantages which make its use undesirable. For instance, laser trimming is relatively slow, due at least in part to the mechanical positioning of the laser during trimming operations. Furthermore, extra mask levels or process steps are typically required in order to fabricate a laser-trimmable component, such as a thin film resistor, thus adding to the fabrication costs and reducing yield. Additionally, since laser trimming is done prior to packaging, any variations due to the packaging process itself cannot be easily corrected.
By way of example, FIG. 1A illustrates a simple pin electronics driver circuit for use in an automated test system. As understood by those skilled in the art, if a current sink 102 and a current source 104, I1 and I2, respectively, are ideal, the rate of change of output voltage, or slew rate (dv/dt), may be easily calculated as                     ⅆ        v                    ⅆ        t              =          i      C        ,
where C is the value of an output ramp-generating capacitor 106 (in Farads) and i is the current value (either I1 or I2, in Amperes) flowing through capacitor 106. Therefore, assuming that the value, C, of capacitor 106 can be accurately defined, the output slew rate (dv/dt) can be accurately predicted.
The pin electronics circuit may also include a switch mechanism 110, or equivalent means, to selectively connect the capacitor 106 to either the current sink 102 (I1) or current source 104 (I2), thereby generating a negative or a positive slew of the output voltage, respectively. An output buffer 108 is also generally connected to the capacitor 106 to provide a low impedance output node, as well as to provide isolation for the high impedance capacitance node 107.
A conventional bipolar junction transistor (BJT) implementation of the current sink 102 is depicted in FIG. 1B. With reference to FIG. 1B, the conventional current sink 102 includes an operational amplifier (op amp) 116 operatively connected in a unity gain closed-loop feedback configuration, with the output 118 of the op amp 116 coupled to the base of a BJT device 112, configured as an emitter follower, and the inverting (xe2x88x92) input 120 of the op amp 116 coupled to the junction of the emitter of the BJT device 112 and a reference resistor 114, which is typically a thin film laser-trimmable resistor. It is well known that the op amp 116 will control the current i by attempting to hold the voltage at the inverting input 120 essentially equal to the voltage at the non-inverting (+) input 122 of the op amp 116. This, in turn, substantially fixes the input voltage +V across the resistor 114. The current i is then adjusted by trimming the resistance R of resistor 114 until a desired current value is obtained. As stated above, as long as the current source 104 and sink 102 are precisely defined, the output voltage slew rate can be accurately predicted and controlled. The adjustment of the resistor value R is conventionally performed by laser trimming.
Conventional means of trimming pin electronics systems do not address the above problems. Accordingly, there is a need in the field of precision semiconductor trimming for a technique that provides quick, easy and accurate adjustment of a semiconductor device or circuit in a cost effective manner, without the need for additional process steps in the fabrication thereof. Furthermore, it would be desirable if such precision trimming capability could be provided after the packaging of the device has been completed.
The present invention provides a method and apparatus for trimming semiconductor devices and circuits, such as pin electronics, which does not employ a laser for the adjustment process. By eliminating the laser trimming operation, the present invention is a cost-effective means of quickly, easily and accurately adjusting a semiconductor device. Moreover, the present invention does not require fabrication of a laser-trimmable component, such as a thin film resistor, that typically requires additional fabrication process steps which can result in a diminished yield. Furthermore, the trimming capability may be provided after completion of the packaging of the device.
In accordance with one embodiment, the present invention addresses the need to precisely adjust a reference voltage or current source by replacing the traditional voltage or current source with a programmable voltage or current source, respectively. In a preferred pin electronics circuit application formed in accordance with the present invention, the conventional current source and sink are preferably replaced by a pair of programmable digital-to-analog (D/A) converters. Each D/A converter is operatively coupled to a select switch or mechanism, such as a multiplexer or suitable equivalent thereof, for selectively connecting the input of the D/A converter to at least one of a data register and a fuse register. The data register is used to temporarily store a digital code word during a first mode of operation of the circuit, preferably a test mode. The fuse register is used to permanently store a desired digital code word when the circuit is in a second mode of operation, preferably a normal mode.
During the test mode of operation, the analog output from the D/A converter is preferably measured while concurrently changing the input digital word stored in the data register. Once the analog output signal is measured to be substantially equal to a predetermined value for the D/A converter output, the digital code word stored in the data register, which corresponds to the desired output, is transferred to the fuse register, which includes a plurality of fusible links, for permanently storing the digital input word to the D/A converter. The digital word is preferably stored in the fuse register by selectively blowing one or more of the fusible links in order to generate the desired digital input word. During normal operation of the circuit, the select switch preferably connects the fuse register to the input of the D/A converter for generating the desired analog output signal.
These and other objects, features and advantages of the present invention will become apparent from the following detailed description of illustrative embodiments thereof, which is to be read in connection with the accompanying drawings.